Most sea going sailboats, and many others, are perfectly capable of sailing around the world, without any fuel. Solar panels are only needed to charge batteries, that are only needed if you want the extras: GPS, radio, other communications. 2 panels and 2 batteries are good enough for most boats, for 1 to 4 persons, traveling for anywhere around 1-2 months across the seas. The limitations are water, and food storage. The solar and batteries are good for years, easily up to 5 years. Humans need other things though. It's not the boats or the technology its the price that limits. (You can buy an ocean cruising bust for 20k USD, easy.) But, it is a life style commitment!